could */*/*/

strong UK [kʊd] / US weak UK [kəd] / US modal verb

Summary:

Could is usually followed by an infinitive without "to": I'm glad you could come. Sometimes it is used without a following infinitive: I came as quickly as I could.
Could does not change its form, so the third person singular form does not end in "-s": She could play the violin when she was six. Questions and negatives are formed without "do": Could he help you? ♦ I could not breathe. The negative form could not is often shortened in conversation or informal writing to couldn't: I couldn't find her phone number.
Could is often used in question tags: They couldn't see us, could they?
Could has no tenses, no participles, and no infinitive form. There is no past tense, but could have followed by a past participle is used for referring to something in the past that was not real, or something that may possibly have been real: I could have been killed. ♦ What was that noise? Could it have been the wind?
Could is used as the past tense of can when it means that someone had the ability to do something, or that something was possible: The Roman army could march 30 miles in a day. But if you want to say that someone actually succeeded in doing something, use was/were able to: Thanks to this grant, we were able to buy new equipment.

1) past tense of "can" used as the past tense of "can"

Renee could already read when she was four.

In the distance I could see a cloud of smoke.

2) spoken used in requests used in polite requests, or when offering or suggesting something

Could I have a glass of water?

Could you post this letter for me?

I wonder if we could borrow your lawnmower?

See:

can I

3) used for saying what is possible

a) used for saying that something is possible or that it may happen

We could still win – the game isn't over yet.

In a situation like this, anything could happen.

could easily:

You could easily lose your way in the dark.

b) spoken used for suggesting to someone a possible course of action, especially when they are in a difficult situation

You could come and stay with us.

could always:

You could always sell the cottage if you need some extra cash.

4) spoken used for expressing annoyance used for showing that you are annoyed at the way someone has behaved

How could you be so cruel!

They could at least have said they were sorry.

5) spoken used for emphasis used for emphasizing how angry, happy, bored etc you are

It's all Helen's fault. I could strangle her!

When I heard the news, I was so happy I could have cried.

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could have (done something) — spoken used for saying that something was possible in the past, even though it did not happen; spoken used for saying that perhaps something was true, although you do not really know

You could have been killed.

I could have told you, but I didn't think you would listen.

She could have married Gerald if she'd wanted to.

The explosion could have been caused by a gas leak.

It could have been Dan, but I'm not sure.

couldn't be better/worse/nicer etc — mainly spoken used for emphasizing that someone or something is extremely good/bad/nice etc

The two brothers couldn't be more different (= they are very different).

I was so nervous, but she couldn't have been nicer to me.

I couldn't ask/wish for... — spoken used for saying that something is so good, you cannot imagine anything better

I couldn't wish for a better husband.

We got fantastic support – we couldn't have asked for more.

- I couldn't

See:

care II